1. Field of the Invention
The present invention relates to a pneumatic radial tire that can improve maneuvering stability while maintaining riding comfort and durability.
2. Description of the Background Art
In recent years, remarkably enhanced equipment and performance of automobiles and well-developed network of roads have been giving rise to demands for tires with constantly stable maneuvering performance. In particular, improvements in maneuvering performance such as braking performance when driving at a high speed, riding comfort and the like are increasingly demanded. As methods for improving braking performance of tires, a method of increasing the rigidity in to and from direction of the tire or the like has conventionally been employed. For example, a method of providing a tread with high rigidity is known.
Japanese Patent Laying-Open No. 01-145205 proposes a radial tire, wherein a ratio Ea*/Eb* of a complex elastic modulus in the tire axial direction Ea* to a complex elastic modulus in the tire radial direction Eb* is set to be greater than 1.5, and wherein a tread is formed using tread rubber preferably including short fibers, in order to attain compatibility between maneuvering stability and riding comfort.
Japanese Patent Laying-Open No. 01-153305 proposes a radial tire aiming to prevent unbalanced wear, deterioration of the appearance and lifetime of the tire and to improve maneuvering stability and braking performance, wherein a groove wall disposed on a groove provided to a tread portion is formed using reinforced rubber. The reinforced rubber is constituted by a rubber base including at least one of natural rubber, polyisoprene rubber, butadiene rubber and polybutadiene based rubber, with which short fibers are compounded. The short fibers are oriented in the depth direction of the groove along the surface of the groove wall.
Japanese Patent Laying-Open No. 06-200076 proposes a radial tire aiming to decrease rolling resistance and to improve maneuvering stability while maintaining riding comfort. The radial tire includes a base tread having a thickness of at most 6 mm. The base tread includes 16-30 parts by weight of short fibers and 30-60 parts by weight of carbon black having an iodine adsorption of 60-130 mg/g relative to total 100 parts by weight of a rubber component.
Japanese Patent Laying-Open No. 11-180108 proposes a pneumatic tire aiming to ensure conductivity with small amount of carbon and further to decrease rolling resistance. In the pneumatic tire, a tread portion is constituted by: a base rubber body arranged internally in the radial direction and formed by a conductive rubber material having a volume specific resistance of less than 1×108Ω; a cap rubber body arranged externally to the base rubber body to thereby form a tread surface and reinforced by silica; and a conductive portion having its inner end coupled to the base rubber body to penetrate the cap rubber body externally in the radial direction, while its outer end forms part of the tread surface. The conductive rubber material includes conductive short fibers wherein reinforcing short fibers are coated by a conductive material to attain conductivity.
Japanese Patent Laying-Open No. 11-255966 proposes a rubber composition for a tire tread aiming to obtain a tire that has less reduction in wear resistance and that is excellent in balance of traction performance, braking performance and cornering performance. The rubber composition is obtained by compounding, relative to 100 parts by weight of a rubber component, 5-45 parts by weight of silica, 0.5-4.0 part by weight of a silane coupling agent and 3-8 parts by weight of a cellulose substance-containing powdery processed article.
Japanese Patent Laying-Open No. 2002-155164 proposes a rubber composition that provides excellent maneuver stability, wear-resistance, wet property and fuel-efficiency. The rubber composition is obtained by compounding, relative to 100 parts: by weight of natural rubber and/or diene type synthetic rubber, 10-120 parts by weight of silica having a cetyltrimethylammonium bromide adsorption specific surface area (CTAB) of 30-135 m2/g, 1-20 parts by weight of organic short fibers having an average diameter of 0.05-50 μm and an average length of 10-2000 μm.
However, for example when short fibers of general organic fibers are compounded, often rubber hardness is small and maneuver stability and durability are reduced, if riding comfort is excellent. Accordingly, an improvement is still necessary in attaining compatibility between riding comfort and maneuver stability.